Semiconductor lasers and optical amplifiers are preferred in transceivers because they are efficiently electrically pumped and the die size is small. Lasing is a radiative recombination process in semiconductors, where an electron in the conduction recombines with a hole in the valance band and a photon is emitted. The reverse process is electron hole pair generation through optical absorption, as occurs in such devices as photodetectors and solar cells.
Silicon photonics is widely seen as an enabling technology to address the exponentially increasing demand for data communication bandwidth. Lasers are critical components in data transmission systems. Two fundamental elements for a laser are its gain medium and resonating cavity. Due to the indirect bandgap of silicon, several approaches of introducing gain medium into the photonic integration material system has been reported, including edge coupled bonding (see, for example A. J. Zilkie, P. Seddighian, B. J. Bijlani, W. Qian, D. C. Lee, S. Fathololoumi, J Fong, R. Shafiiha, D. Feng, B. J. Luff, X. Zheng, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “Power-efficient III-V/Silicon external cavity DBR lasers,” Optics Express, Vol. 20, pp. 23456-23462, 2012; S. Tanaka, S. H. Jeong, S. S., T. Kurahashi, Y. Tanaka, and K. Morito, “High-output-power, single-wavelength silicon hybrid laser using precise flip-chip bonding technology,” Optics Express, Vol. 20, pp. 28057-28069, 2012), direct bonding (see, for example A. W. Fang, H. Park, O. Cohen, R. Jones, M. J. Paniccia, and J. E. Bowers, “Electrically pumped hybrid AlGaInAs-silicon evanescent laser,” Optics Express, Vol. 14, pp. 9203-9210, 2006; S. Keyvaninia, G. Roelkens, D. Van Thourhout, C. Jany, M. Lamponi, A. Le Liepvre, F. Lelarge, D. Make, G. H. Duan, D. Bordel, and J. M. Fedeli, “Demonstration of a heterogeneously integrated III-V/SOI single wavelength tunable laser,” Optics Express, Vol. 21, pp. 3784-3792, 2012; T. Creazzo, E. Marchena, S. B. Krasulick, P. Yu, D. Van Orden, J. Y. Spann, C. C. Blivin, L. He, H. Cai, J. M. Dallesasse, R. J. Stone, and A. Mizrahi, “Integrated tunable CMOS laser,” Optics Express, Vol. 21, pp. 28048-28053, 2013), heavily N-doped germanium (see, for example R. E. Camacho-Aguilera, Y. Cai, N. Patel, J. T. Bessette, M. Romagnoli, L. C. Kimerling, and J. Michel, “An electrically pumped germanium laser,” Optics Express, Vol. 20, pp. 11316-11320, 2012), and quantum dot structures (see, for example T. Wang, H. Liu, A. Lee, F. Pozzi, and A. Seeds, “1.3-μm InAs/GaAs quantum-dot lasers monolithically grown on Si substrates,” Optics Express, Vol. 19, Issue 12, pp. 11381-11386 (2011)). Laser cavities are usually built using Distributed Bragg Reflectors (DBRs), or DBRs together with ring resonator filters, which require high lithography resolution and are sensitive to fabrication variations.
Conservation of both energy and momentum are required in the lasing process. Energy conservation is satisfied because the emitted photon's energy is equal to the bandgap of the semiconductor. However, a photon's momentum is negligible compared to that of an electron or a hole. To meet the momentum conservation requirement, the top of the valence band and the bottom of the conduction band need to be aligned. In other words, the semiconductor needs to be a direct bandgap material. A number of III-V compound materials such as gallium arsenide (GaAs) and indium phosphide (InP) are direct bandgap semiconductors. However, silicon is an indirect bandgap semiconductor.
Raman silicon lasers have been demonstrated. Making an electrically pumped silicon laser is prohibitively difficult. Another gain material has to be introduced into the silicon material system in which laser action is desired. Various gain integration approaches have been reported, including monolithic epitaxy, wafer bonding, and SOA edge coupling.
X. Shu, S. Jiang, and D. Huang, “Fiber grating Sagnac loop and its multiwavelength-laser application,” IEEE Photonics Technology Letters, Vol. 12, pp. 980-982, 2000 is said to describe a novel simple comb filter, which is based on a Sagnac interferometer with a fiber Bragg grating asymmetrically located in its fiber loop. The filter has advantages of simple design and easy fabrication, low insertion loss and low cost. Two filters with triple bandpasses and dual bandpasses, respectively, were fabricated and applied to an erbium-doped fiber ring laser. Stable triple-wavelength and dual-wavelength laser operations have been demonstrated.
J. Zhou, P. Yan, H. Zhang, D. Wang, and M. Gong, “All-fiber mode-locked ring laser with a Sagnac filter,” IEEE Photonics Technology Letters, Vol. 23, pp. 1301-1303, 2011 is said to describe the following: Terbium-doped mode-locked fiber lasers are versatile sources of femtosecond pulses. The development of new pulse-shaping mechanisms in fiber lasers allows the generation of higher energy femtosecond pulses than the soliton mode-locked lasers which are required in the application. However the pulses from Yb-doped mode-locked fiber lasers are longer in width than the soliton pulses due to the normal dispersion of the fiber. Thus grating pairs are necessary in and out of the cavity to provide negative dispersion. An All-Normal-Dispersion (ANDi) Yb-doped fiber laser has been demonstrated with a spectral filter instead of the grating pair in the cavity. The spectral filtering of a highly-chirped pulse in the laser cavity is the key component of the pulse shaping in this type of mode-locked laser. The influence of the bandwidth of the filter on the mode-locking has been theoretically investigated. The bulk interference or birefringent filter is commonly used as the spectral filter. In order to develop the all-fiber configuration free from misalignment some fiber-type filters have been investigated. Recently an all-fiber Lyot filter with a section of Polarization Maintaining (PM) fiber has been used as the spectral filter in ANDi laser and 240 fs dechirped pulses were obtained. According to the discussion in [2], [3] the duration of the mode locked pulse diminishes with the decrease of the filtering bandwidth until the mode-locking fails. The Lyot filter can be used as a bandwidth-tunable filter as the bandwidth depends on the length of the PM fiber. However the modulation depth of the Lyot filter is dominated by the Polarization Controllers (PCs).
There is a need for an improved external cavity for use with lasers.